The present invention relates to an apparatus for applying a coating liquid to a belt-like support (web) while the support in continuously moving. More particularly, the invention relates to an improved coating rod used in a coating apparatus.
Various apparatuses have been known for applying a coating liquid to a continuously travelling web. Such apparatuses generally employ a step (referred to as an application system) in which the coating liquid is transferred to the web and a step (referred to as a metering system) in which the coating liquid transferred to web is metered so as to obtain a desired coating thickness.
Coating methods are generally grouped and classified on the basis of differences in the application system and the metering system employed. Known application systems include a roller coating method, a dip coating method, and a fountain coating method. Known metering systems include an air-knife coating method, a blade coating method, and a rod coating method.
In the rod coating method, an excess quantity of the coating liquid is transferred to the web, and the surplus coating liquid is removed by a stationary or rotating rod, thereby to obtain a desired quantity of coating liquid on the web. The rod coating method is widely used due to the advantage that a thin-film coating can be effected at a high speed using a simple coating apparatus structure and employing a simple operation. Substantially any application system can be used with the rod coating method. However, the roller coating method, particulary a roller coating method known as the "kiss-roller" coating method, is most generally used because of its simplicity.
In the conventional rod coating method, however, the application and metering systems are performed completely separately from one another. This is disadvantageous in that the conditions for the two systems must be set entirely separately from one another, thereby making the overall coating operation complex. Also, the overall coating apparatus requires a substantially large installation space, reducing the economy of the operation.
A coating apparatus has been proposed, for example, in Japanese Patent Publication No. 36529/1977, in which a physical space is mechanically formed in a formed by closely winding a wire around the surface of a rod (hereinafter, referred to as a "wire bar") portion by a coating applicator, a heat-resistant filler, and a thin guide plate so as to form a wedge-like liquid well. Also, a coating apparatus has been disclosed in U.S. Pat. No. 4,263,870 in which a bar coating process for coating a coating liquid on a continuously travelling web which comprises the steps of supplying a coating liquid so as to form a liquid reservoir immediately before a position of contact between a bar and the web, and coating the coating liquids on the web using the bar, wherein the bar is axially positioned perpendicularly to the travelling direction of the web; is supported on a supporting member and is rotated in the same direction as that of the web while coming into contact with the web. These two apparatuses are improved over the earlier coating apparatuses in that their operation is made easier, no large installation space is required, and a coating film having superior surface properties can be formed.
On the other hand, in these apparatuses adjustment of the quantity of the applied coating liquid is performed by changing the diameter of the wire on the coating rod, the wire being wound around an inner cylindrical base member of the rod. To reduce the amount of coating liquid, the diameter of the wire is reduced. However, from the viewpoint of manufacturing accuracy and durability, there is a lower limit to the wire diameter, thus making it very difficult to form a very thin coating layer. Also, the wire has a tendency to wear or suffer damage over time, thereby generating undesirable variations or faults in the coated layer.